Open lamp voltage and short circuit protection schemes are often required in cold cathode fluorescent lamp (CCFL) inverter applications for safety and reliability reasons. In an open lamp condition, there might be a very large undesirable voltage occurring across the outputs if protections are not in place. This undesirable voltage may be several times higher than a nominal output and could be harmful to circuit components. In a shorted lamp condition, a protection circuit is desired to reduce the power level or to shut down the circuit completely to avoid circuit breakdown or other possible catastrophic situations.
A conventional method to achieve open lamp voltage protection is to monitor the winding current. A problem with this approach is that the winding current is not significantly reduced in an open lamp event because of large circulating energy. A conventional method to achieve shorted lamp protection is to measure capacitor voltage. In the shorted lamp condition, the voltage gain has a dramatic drop and it can be used as an indication of the short circuit condition. However, in a conventional CCFL inverter, open lamp voltage protection and shorted lamp protection are completely separate circuits. In order to achieve both open lamp protection and shorted lamp protection, not only are two sets of independent circuits necessary, but also two separate sets of pins are needed in the controller circuit. This results in unwanted complexity of the overall circuit and associated increase costs.